This invention relates to a device for controlling the travel distance of a chisel for a point feed system for an electrolytic cell designed for aluminum production.
The units usually used enable the addition of alumina and/or electrolyte to one or several feed points per cell; these products are added into the electrolytic bath through a hole made by a crust breaker that is lowered periodically and breaks the crust or keeps the hole open.
The travel distance of the crust breaker is normally equal to a fixed length determined by the mechanical system that moves it vertically.
This type of device has disadvantages. Depending on the hardness of the crust and the level of the free surface of the electrolyte, it sometimes occurs that the travel distance of the crust breaker is too short to break the crust so that alumina can be added. On the contrary, if this travel distance is too great, it is possible that the active end of the crust breaker, also called the chisel, remains in the electrolytic bath for too long. It has also been observed that the chisel can penetrate deeply into the electrolyte. In this case, the chisel carries part of the solidified bath with it in the form of a deposit that increases every time that the crust breaker is lowered. Prolonged contact between the chisel and the electrolyte degrades the chisel due to the high temperature and the chemically aggressive nature of the bath.
Furthermore, since the chisel is guided by a sheath, the solidified bath deposit on the chisel may form an accumulation despite the presence of a scraper that could make it impossible for the chisel to rise all the way up in the sheath. The result can be that the chisel gets blocked in the device, causing closure of the alumina feed and/or electrolyte feed hole.
This jamming and blocking phenomenon can also cause the chisel to break and/or wear, mechanical shocks due to the increase in tension of the jack controlling the crust breaker movement, and degradation of the material used as electrical insulation due to thermal shocks and lateral forces that occur when the solidified bath comes into contact with the scraper. The device can then no longer function.
It is also difficult to control penetration of the chisel into the electrolyte due to the variation of the electrolyte level, particularly resulting from operations carried out on the pot and variations in the distance between the anodes and the metal caused particularly by regulation of the resistance of the electrolytic cell.
Feeding means to supply alumina to an electrolytic cell producing aluminum, particularly according to the Hall-Héroult process, have been described in documents FR 2 483 965, FR 2 614 320, U.S. Pat. No. 4,563,255 and WO 0106039.
Documents FR 2 483 965 and WO 0106039 each describe a device used to create an electrical contact to raise the chisel when it comes into contact with the electrolyte. However, these documents do not describe any means of establishing such a contact.
Document U.S. Pat. No. 4,563,255 describes a device of the same type as the above, more specifically related to the structure of the electrical circuit for detection of contact between the chisel and the electrolyte, but does not indicate any precise means for creating contact on the chisel, since this contact is simply diagrammatically shown at the crust breaker actuation jack.
Document FR 2 614 320 divulges how to detect contact between the chisel and the electrolytic bath by means of an electrical circuit connected the chisel rod to the cathode substrate. However, in this document the electrical circuit is connected to the chisel rod through an electrical contact subject to friction that can fluctuate; such a sliding contact cannot reliably close the electrical contact on the chisel rod. Consequently, contact between the chisel and the electrolytic bath is not detected with certainty and in good time, and the chisel can remain immersed in the electrolytic bath for longer than necessary, causing the clogging phenomenon mentioned above.